135 research outputs found
Efficient collinear third-harmonic generation in a single two-dimensional nonlinear photonic crystal
We propose novel multi-phase-matched process that starts with generation of a
pair of symmetric second-harmonic waves. Each of them interacts again with the
fundamental wave to produce two constructively interfering third harmonic waves
collinear to the fundamental input wave.Comment: Summary of presentation at the IQEC/LAT-2002 conferenc
High quality anti-relaxation coating material for alkali atom vapor cells
We present an experimental investigation of alkali atom vapor cells coated
with a high quality anti-relaxation coating material based on alkenes. The
prepared cells with single compound alkene based coating showed the longest
spin relaxation times which have been measured up to now with room temperature
vapor cells. Suggestions are made that chemical binding of a cesium atom and an
alkene molecule by attack to the C=C bond plays a crucial role in such
improvement of anti-relaxation coating quality
Pressure broadening and shift of D1 line of Ag by He, Ar and N2
We have studied experimentally pressure broadening and shift of Ag D1 line
caused by He, Ar and N2 buffer gases. The measurements were done in a heat-pipe
type absorption cell at a temperature of ~1000 K and gas pressures up to 1000
torr. The measured values for pressure broadening and shift (in MHz/Torr) are
as follows: Ag-He 5.8(1), +1.16(2); Ag-Ar 5.2(2), -2.28(4); Ag-N2 5.2(1),
-2.52(7). The "+" and "-" signs indicate the direction for the shifts to the
blue and red side of the spectrum, respectively.Comment: 4 pages, 4 figure
Investigation of Anti-Relaxation Coatings for Alkali-Metal Vapor Cells Using Surface Science Techniques
Many technologies based on cells containing alkali-metal atomic vapor benefit
from the use of anti-relaxation surface coatings in order to preserve atomic
spin polarization. In particular, paraffin has been used for this purpose for
several decades and has been demonstrated to allow an atom to experience up to
10,000 collisions with the walls of its container without depolarizing, but the
details of its operation remain poorly understood. We apply modern surface and
bulk techniques to the study of paraffin coatings, in order to characterize the
properties that enable the effective preservation of alkali spin polarization.
These methods include Fourier transform infrared spectroscopy, differential
scanning calorimetry, atomic force microscopy, near-edge X-ray absorption fine
structure spectroscopy, and X-ray photoelectron spectroscopy. We also compare
the light-induced atomic desorption yields of several different paraffin
materials. Experimental results include the determination that crystallinity of
the coating material is unnecessary, and the detection of C=C double bonds
present within a particular class of effective paraffin coatings. Further study
should lead to the development of more robust paraffin anti-relaxation
coatings, as well as the design and synthesis of new classes of coating
materials.Comment: 12 pages, 12 figures. Copyright 2010 American Institute of Physics.
This article may be downloaded for personal use only. Any other use requires
prior permission of the author and the American Institute of Physics. The
following article appeared in the Journal of Chemical Physics and may be
found at http://link.aip.org/link/?JCP/133/14470
Unc5B Interacts with FLRT3 and Rnd1 to Modulate Cell Adhesion in Xenopus Embryos
The FLRT family of transmembrane proteins has been implicated in the regulation of FGF signalling, neurite outgrowth, homotypic cell sorting and cadherin-mediated adhesion. In an expression screen we identified the Netrin receptors Unc5B and Unc5D as high-affinity FLRT3 interactors. Upon overexpression, Unc5B phenocopies FLRT3 and both proteins synergize in inducing cell deadhesion in Xenopus embryos. Morpholino knock-downs of Unc5B and FLRT3 synergistically affect Xenopus development and induce morphogenetic defects. The small GTPase Rnd1, which transmits FLRT3 deadhesion activity, physically and functionally interacts with Unc5B, and mediates its effect on cell adhesion. The results suggest that FLRT3, Unc5B and Rnd1 proteins interact to modulate cell adhesion in early Xenopus development
Multistep Parametric Processes in Nonlinear Optics
We present a comprehensive overview of different types of parametric
interactions in nonlinear optics which are associated with simultaneous
phase-matching of several optical processes in quadratic nonlinear media, the
so-called multistep parametric interactions. We discuss a number of
possibilities of double and multiple phase-matching in engineered structures
with the sign-varying second-order nonlinear susceptibility, including (i)
uniform and non-uniform quasi-phase-matched (QPM) periodic optical
superlattices, (ii) phase-reversed and periodically chirped QPM structures, and
(iii) uniform QPM structures in non-collinear geometry, including recently
fabricated two-dimensional nonlinear quadratic photonic crystals. We also
summarize the most important experimental results on the multi-frequency
generation due to multistep parametric processes, and overview the physics and
basic properties of multi-color optical parametric solitons generated by these
parametric interactions.Comment: To be published in Progress in Optic
Structural basis for the cooperative DNA recognition by Smad4 MH1 dimers
Smad proteins form multimeric complexes consisting of the ‘common partner’ Smad4 and receptor regulated R-Smads on clustered DNA binding sites. Deciphering how pathway specific Smad complexes multimerize on DNA to regulate gene expression is critical for a better understanding of the cis-regulatory logic of TGF-β and BMP signaling. To this end, we solved the crystal structure of the dimeric Smad4 MH1 domain bound to a palindromic Smad binding element. Surprisingly, the Smad4 MH1 forms a constitutive dimer on the SBE DNA without exhibiting any direct protein–protein interactions suggesting a DNA mediated indirect readout mechanism. However, the R-Smads Smad1, Smad2 and Smad3 homodimerize with substantially decreased efficiency despite pronounced structural similarities to Smad4. Therefore, intricate variations in the DNA structure induced by different Smads and/or variant energetic profiles likely contribute to their propensity to dimerize on DNA. Indeed, competitive binding assays revealed that the Smad4/R-Smad heterodimers predominate under equilibrium conditions while R-Smad homodimers are least favored. Together, we present the structural basis for DNA recognition by Smad4 and demonstrate that Smad4 constitutively homo- and heterodimerizes on DNA in contrast to its R-Smad partner proteins by a mechanism independent of direct protein contacts
Controlling atomic vapor density in paraffin-coated cells using light-induced atomic desorption
Atomic-vapor density change due to light induced atomic desorption (LIAD) is
studied in paraffin-coated rubidium, cesium, sodium and potassium cells. In the
present experiment, low-intensity probe light is used to obtain an absorption
spectrum and measure the vapor density, while light from an argon-ion laser,
array of light emitting diodes, or discharge lamp is used for desorption.
Potassium is found to exhibit significantly weaker LIAD from paraffin compared
to Rb and Cs, and we were unable to observe LIAD with sodium. A simple LIAD
model is applied to describe the observed vapor-density dynamics, and the role
of the cell's stem is explored through the use of cells with lockable stems.
Stabilization of Cs vapor density above its equilibrium value over 25 minutes
is demonstrated. The results of this work could be used to assess the use of
LIAD for vapor-density control in magnetometers, clocks, and gyroscopes
utilizing coated cells.Comment: 10 pages, 11 figure
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